The present invention relates in general to program logic controls abbreviated PLC's, and in particular to a new and useful PLC which discriminates between working and non-working days whether the non-working days are holidays or weekends.
There are many control applications that require supervisory set point control that is a function of time-of-day only. Some examples include the generation of temperature set points for building heating, ventilating and air conditioning control, the operation of mechanical equipment, such as water pumping, during off-peak hours to take advantage of reduced utility rates during non-peak demand, and generation of feed forward chemical demand in waste water treatment operations where diurnal or daylong variations are known but key process variables are difficult or expensive to measure (e.g. coagulant demand and chlorine demand for waste water treatment).
Typical diurnal variations for building temperature settings and waste water treatment coagulant demand are shown in FIGS. 1 and 2. These control applications require set point or control output variation on an hourly basis and differentiation among working days, weekends and holidays.
Programmable Logic Controllers have been used in applications requiring timing, sequence, logic, counting and arithmetic to control machines and processes. The PLC is well suited for handling sequential control ahd counting. Configurations of PLC's to handle function generators, analog output signals, and precision time integration on the time scale of days to weeks is very complex, however.
For example, Allen Bradley Bulletin 1774 discusses an arithmetic module (Cat. No. 1774-102) which provides four function arithmetic capabilities (+,-,.times. and .div.) of three digit decimal numbers (3 digits for + and -) (6 digit resultant for .times. and .div.). The capabilities of the Bradley function generators and precision time integrators, on a time scale of days to weeks, is not available.
Modicon 584 also provides basic arithmetic operations (+,-,.times. and .div.) with integer arithmetic. The division operation maintains whole number and decimal fraction. A move instruction is provided which could crudely perform an analog transfer or function generator operation.